In electrophotography, electrostatic printing or electrostatic recording, the latent image on a photoreceptor is conventionally developed by either of the following two processes: liquid development which uses a liquid developer having fine pigment or dye particles dispersed in a dielectric organic carrier liquid, and dry development which uses a powder developer made of a toner having carbon black or other colorant particles dispersed in a natural or synthetic resin binder, and dry development processes include cascade development, fur brush development, magnetic brush development, impression development and powder cloud development. The dry-developed image is optionally transferred to a receiving sheet, usually paper, and fixed thereto.
In conventional copying machines, the toner image is fixed by pressure alone, or by exposure to solvent vapors or by heating it to fuse to the paper. The third method is conventionally referred to as the thermal fixing process and there are two types: noncontact fusing using an electric oven and contact fusing using heated rollers. Of these fixing methods, fixing by contact fusion is most often used since it achieved high heat efficiency and is suited to electrophotographic copiers and other transfer-type recording apparatuses that are designed for high-speed copying. But this method has several defects: the fixing unit consumes the most power of all the components of the copier; the fixing unit requires heating means; and long warm-up time is necessary, or it takes long for the fixing unit to become "ready" following power application. To eliminate these defects, the pressure fixing method has been proposed. Since the fixing rollers used in this method are not heated, the following advantages are obtained: use of less energy, no pollution hazard, no warm-up time, no chance of the copy to be scorched, high-speed fixing and simplified construction of the fixing unit. These advantages make the pressure fixing mehtod particularly suitable for general-purpose copiers. For all these advantages, however, this method has one great problem, insufficient fixing ability, since, unlike the thermal fusion process wherein the toner is fixed in a molten state, the toner particles are simply crushed (deformed plastically) and forced into the receiving paper.
In the fixing unit, unfixed toner particles are transferred to the surface of the fixing rollers and are replaced on the next coming receiving sheet to foul the toner image on it. This phenomenon is usually called "offset phenomenon" and in the contact fusing method, crosslinking the polymer that is one component of the toner with a crosslinking agent or incorporating a polyolefin of a relatively low weight average molecular weight in the toner is known to be effective for preventing offset phenomenon. But in the pressure fixing method, a far greater pressure is applied to the toner image and metal rollers having great surface energy and poor release properties and used, so the chance of offset phenomenon is greater than in the contact fusing method. What is more, this pressure fixing method has smaller fixing ability than the contact fusing method. Therefore, the toner or antioffset phenomenon means designed for the contact fusing method cannot be directly applied to the pressure fixing method. In particular, the toner for developing an electrostatic latent image that can be used in the pressure fixing method must have not only good fixability but also long keeping quality, great durability, high resistance to moisture and good pictorial rendition.